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Renewable first wall coatings in operating plasma devices
Journal of Nuclear Materials 417 (2011) 586–591
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Renewable first wall coatings in operating plasma devices O.I. Buzhinskij a,⇑, V.A. Barsuk a, V.P. Logvinenko b, A.I. Mescheryakov b, V.G. Otroshchenko a a b
Troitsk Institute for Innovation and Fusion Research, Troitsk, Moscow Reg. 142190, Russia Prokhorov General Physics Institute, Vavilova St. 38, Moscow, 119991, Russia
a r t i c l e
i n f o
Article history: Available online 30 March 2011
a b s t r a c t Experimental results on boronization in plasma shots at the T-11M tokamak and stellarator L-2M are presented. Nontoxic and nonexplosive metacarborane C2H12B10 was used in boron deposition process. Experiments at the tokamak have been carried out in shots with parameters: toroidal field 1–1.2 T, plasma current Ip = 70 kA, average shot duration tp 150 ms and electron density along the central chord ne 2.5 1013 cm 3. The impurities in wall areas have been suppressed. High vacuum characteristics of the discharge chamber were stabilized. Stabilization of a plasma filament has improved. Experiments at the stellarator have been carried out in ohmic heating discharges with parameters: plasma current Ip = 18–20 kA, toroidal magnetic field Bo = 1.25 T, averaged electron density ne = (1.0–1.2) 1013 cm 3, central electron temperature Te = 300 eV and shot duration tp = 30–40 ms. High repeatability of experimental results was achieved. The technology developed provides an opportunity for practical production of renewable boron–carbon coatings using plasma shots in large-scale tokamaks and stellarators. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction In fusion facilities with magnetic confinement the deposition of boron-containing films is widely used for decreasing impurities in the plasma [1,2]. Boronization is carried out in low density helium glow discharges or with the use of plasma electron-cyclotron heating [2]. It is established that the boron film improves conditions of the vacuum chamber surface and plasma facing components. Boron-carbide films decrease the level of heavy impurities (iron, chromium, nickel), they actively absorb oxygen, substantially decrease carbon content and protect the chamber surfaces contacting the plasma. However, the use of a glow discharge with sufficiently low energy characteristics only allows amorphous film thickness of 100 nm, which is eroded during shots of total duration 1–10 s. The typical erosion rate of this film is 3–10 nm/s [2]. Using relatively high density and temperature (40 eV) of the plasma, boron-carbide films with composition close to stoichiometric B4C have been obtained in the PISCES-B facility in the University of California, San Diego [3]. Deposition was performed in plasmas with parameters similar to the DIII-D tokamak divertor plasma. Nontoxic and nonexplosive metacarborane C2H12B10 was used as the working substance. The deposition rate was extremely high and achieved 30 nm/s, approximately one thousand times the rate of film deposition in a glow discharge. Thickness of the deposited layer depended on discharge time and 40 lm thickness was achieved. The coating was dense, without pores and had high hard⇑ Corresponding author. Tel.: +7 496 7510538; fax: +7 495 3345510. E-mail addresses: [email protected] (O.I. Buzhinskij), [email protected] (A.I. Mescheryakov). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.03.038
ness, good adhesion to sample surfaces. The high film deposition rates are apparently linked to the high degree of carborane ionization and dissociation caused by the high density 40 eV PISCES-B plasma compared to electron temperature <1 eV in glow discharge plasmas. Therefore, it is of interest to investigate boronization of the discharge chamber in tokamak and stellarator plasma shots with higher density and electron temperatures. A new method of boron-carborane film deposition was developed for that purpose. The boronization is carried out during the operating pulses of plasma devices when the plasma is heated ohmically and carborane is injected into the plasma during the ohmic discharge. We called this the pulsed boronization method. The present work gives the results of experimental studies of pulsed boronization in ohmic discharge plasmas at the T-11M tokamak [4] and the L-2M stellarator using carborane C2B10H12.
2. Experiments on the T-11M Boronization of the tokamak T-11M chamber during plasma shots was carried out after experiments with the lithium limiter [5]. The placement of the lithium and graphite limiters, diagnostics, transport device with reference specimens and the carborane container in the T-11M tokamak is shown in Fig. 1. The boronization experiments was used nontoxic, nonexplosive metacarborane (C2H12B10) as the working agent. Carborane is a clear crystal substance at room temperature. The molecular structure of carborane is dodecahedral, built from boron and carbon atoms, surrounded by hydrogen atoms. The melting point of metacarborane is 263–265 °C and temperature of sublimation 20 °C. Crystalline
O.I. Buzhinskij et al. / Journal of Nuclear Materials 417 (2011) 586–591
587
Fig. 1. Location of diagnostics (LiI, LiII, Ha, SRX, IR), carborane container, and limiters on the T-11M tokamak.
metacarborane was placed in the glass container that was connected to the high-vacuum electromagnetic valve. The electromagnetic valve was connected to a diagnostic port flange of the tokamak T-11M discharge chamber through a vacuum gate valve. The temperature of the container was regulated by a special heater in the range room temperature to 150 °C. The electromagnetic valve was controlled by the tokamak toroidal field with the time of its opening and closing variable over a wide range. The tokamak plasma current was stabilized at 70 kA, and the toroidal field on axis 1.15–1.2 T. A deuterium and hydrogen mixture (H/D ratio 1:1) was used in plasma shots. The lithium limiter was moved away to the ‘‘shadow’’ during operation with a graphite limiter located at 20 cm from the plasma center. Behavior of the lithium, carbon, boron, deuterium and hydrogen light emission lines was determined by an optical method. Boronization in the T-11M tokamak was carried out after operation with the lithium limiter, without any preliminary induction heating or cleaning of the chamber by a glow discharge. In the preliminary discharges (before boronization) atomic lithium and impurities entered the plasma, produced by ion sputtering from chamber walls. Plasma radiation spectrum (Li, B, C and impurity ion lines and H, D lines) was analyzed by a high resolution optical spectrometer. In Fig. 2 the radiation spectrum of the plasma before boronization is shown by curve 2. There is a bright Li ion line in the plasma radiation spectrum. However, after several shots with carborane the Li line and impurities lines practically vanish
from plasma radiation spectrum (curve 1 in Fig. 2), and a B ion line appears. The plasma radiation spectrum also contains deuterium and hydrogen lines. During lithium experiments the D:H ratio in peripheral plasma was 1:1.3. During boronization the D:H ratio in the peripheral plasma decreased to 1:4 (Fig. 2). When the valve for carborane injection was closed, the B line vanished from the plasma radiation spectrum, at the same time lines from impurities completely vanished. The carborane injection valve was opened for at most 50 ms before the plasma shots and the time of its opening could be varied over a wide range. Change of the valve opening time was easily detected by optical diagnostics of the radiation spectrum during plasma shots. Injection of carborane in the tokamak plasma shots improved the stabilization of plasma filaments, and hydrogen recycling from the chamber walls was decreased. The duration of the tokamak plasma shots increased from tp = 150 to 250 ms at the density of ne = 4.65 1013 cm 3 and up to 350 ms at a density of Ie = 1.3 1013 cm 3. Boronization resulted in a considerable decrease of the volt–second consumption rate (and the corresponding increase of shot duration) (Fig. 3). The impurities near the wall were suppressed and the high vacuum of the discharge chamber was stabilized. Concentration of impurities had decreased by 3–5 times, so the radiation losses decreased by three times and amounted to only 10–20% of the input energy. The boronization during tokamak plasma shots formed a boron–carbon coating on reference specimens and the AISI 321 stainless steel first wall of the discharge chamber (Fig. 4). Scanning electron and optical microscopy examination of coating surface on the samples showed they had a cellular microstructure. The size of cells in the centre of a sample was less than at the periphery. In the centre of a sample the coating thickness was 200 nm, and at the periphery 120 nm. Boronization in tokamak plasma shots occurred during about 50 pulses with total exposure time 8 s, thus the film deposition rate was 25 nm/s, close to the rate of the boron film deposition in the PISCES-B experiments. All plasma shots with carborane had similar values of main plasma characteristics. Vickers microhardness measurements with a load of 10 g showed hardness of the crystalline Si substrate to be H10-250 and microhardness of the boron film H10-600, this indicates the coating produced have ordered structure. (For crystalline boron carbide, B4C, produced by chemical vapor deposition, H101800.)
3. Experiments on the L-2M stellarator
Fig. 2. Plasma radiation spectrum of the T-11M tokamak, before boronization (with lithium limiter) – shot 22641 and at the beginning of boronization – shot 22673.
Glow discharge boronization of the vacuum chamber walls has been carried out on the L-2M stellarator since 2002, and has allowed considerable improvement of the discharge parameters
588
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Fig. 3. Plasma shot parameters before (curve 1), during (curve 2) and after (curve 3) boronization at the carborane temperature of 100 °C: a – current, b – density, c – volt– seconds, d – hard X-ray.
[6]. To carry out pulsed boronization, the same boronization chamber as for the glow boronization was used (see Fig. 5). The only difference is that the output valve was replaced by a pulsed admission valve (5). The boronization chamber was heated to 110 °C and the glass carborane container (2) was heated to 60 °C. The pressure near the pulsed admission valve was maintained at 8 10 2 torr. The pulsed boronization was carried out after preliminary preparation of the stellarator chamber. The magnetic system of the stellarator produces a divertor configuration of the magnetic field with two zero points (see Fig. 6). The flux of charged particles and energy at the X-point of the separatrix are ten times larger than in other locations of the plasma surface. Thus the boron-carbide film is expected to be preferentially formed near the X-point of the separatrix. In plasma shots the plasma-wall interaction occurs precisely at these locations of the chamber surface, so the bor-
on-carbide coating in these locations plays the most important role. As a result of the pulsed boronization cycle which includes the preliminary glow boronization, the fluorescence intensity of carbon and oxygen lines during operation with a hydrogen plasma became 3–5 times lower than before boronization (Fig. 7), and the total radiation losses 2–3 times lower. The loop voltage drops by a factor of 1.5–2 within the constant current ohmic heating. This shows the decrease of plasma effective charge. The plasma density remains constant during all the pulses of ohmic discharge after boronization. It is also worth mentioning that the amount of carbon and oxygen impurities and also the radiation losses remain practically the same after pulsed boronization as after glow boronization. The life test of the deposited boron-carbide film has shown that the film starts to degenerate after approximately 200 stellarator
O.I. Buzhinskij et al. / Journal of Nuclear Materials 417 (2011) 586–591
589
50 ms operating pulses (thus total time of ohmic discharge on the film was 10 s). That is, the film formed by pulsed boronization is more resistant to dispersion than the film formed by the glow boronization. The degradation rate was approximately half that of the glow discharge-formed surface. Apparently boron-carbide film of higher density is formed at those segments of the vacuum chamber surface which are close to the X-point of the separatrix. The ion flux from plasma near Xpoints is 10 times greater than the mean flux from plasma to the segments of the wall which are far from these points. The porous film formed as a result of usual glow boronization remains on the other areas of the vacuum chamber surface. 4. Conclusions
Fig. 4. Boron–carbon coating of the T-11M tokamak discharge chamber.
Experiments on boronization of the T-11M tokamak and the L2M stellarator during plasma shots used metacarborane were successful. As a result of boronization in the tokamak, a film with a
Fig. 5. Boronization system of vacuum chamber on the stellarator L-2M: 1 – silicogel pump, 2 – carborane container, 3 – thermocouple tube, 4 – the cross section of stellarator chamber, 5 – pulsed valve.
Fig. 6. Magnetic surfaces in the stellarator L-2M.
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Fig. 7. Ohmic discharge before (curve 1) and after (curve 2) of pulsed boronization in the stellarator L-2M.
Vicker’s microhardness H10-600 and thickness up to 0.2 lm at a deposition rate 25 nm/s was produced. Impurities in the wall zone were suppressed and a stable, high vacuum was maintained in the discharge chamber. Stabilization of plasma filaments improved, and hydrogen recycling from the chamber walls decreased. Boronization resulted in a considerable decrease of the volt–seconds consumption rate (and, correspondingly, to an increase of shot duration). Plasma shot duration without disruptions increased substantially and at a density of ne = 1.3 1013 cm 3, Ip = 70 kA in tokamak was 350 ms and at a density of ne = 4.65 1013 cm 3, Ip = 70 kA was 250 ms. Pulsed boronization of the vacuum chamber after preliminary preparation by glow boronization was carried out in the L-2M stellarator. Comparison of the results between the pulsed boronization and the glow boronization shows: (1) The amounts of carbon and oxygen impurities in the hydrogen plasma decrease by 3–5 times after boronization and was practically the same after pulsed boronization or glow boronization.
(2) The pulsed boronization applies the boron-carbide film of higher density on the vacuum chamber walls of the stellarator compared to the result with glow boronization. The total time that the wall protection remained effective was two times greater compared to the film formed during the glow boronization. A high repeatability of plasma shot characteristics was achieved in both devices. Proposed use of this technology opens the possibility of practical production of renewable boron carbide coating using plasma shots in large-scale tokamaks and stellarators, such as DIII-D, JET, JT-60 UP, LHD, ITER, and DEMO.
Acknowledgements The authors express their gratitude to Prof. Azizov E.A. for fruitful discussions, permanent interest and support. Work supported by RFBR Grant 11-02-01341-a and in part by RFBR Grant 08-0800504-a.
O.I. Buzhinskij et al. / Journal of Nuclear Materials 417 (2011) 586–591
References [1] J. Winter, H.G. Esser, L. Konen, et al., J. Nucl. Mater. 162–164 (1989) 713. [2] O.I. Buzhinskij, Yu.M. Semenets, J. Fusion Technol. 32 (1) (1997) 1. [3] O.I. Buzhinskij, V.A. Barsuk, V.G. Otroshchenko, J. Nucl. Mater. 390–391 (2009) 996.
591
[4] O.I. Buzhinskij, V.G. Otroshchenko, D.G. Whyte, et al., J. Nucl. Mater. 313–316 (2003) 214. [5] V.A. Evtikhin, I.E. Lyublinski, A.V. Vertkov, et al., The liquid lithium fusion reactor, in: Proceeding of 16th International Conference on Fusion Energy, Montreal, 7–11 Oct. 1996. Fusion Energy 1997, IAEA, Vienna, 1997, vol. 3, pp. 659–665. [6] A.I. Mescheryakov, D.K. Akulina, Plasma Phys. Rep. 31 (6) (2005) 452.
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Renewable first wall coatings in operating plasma devices O.I. Buzhinskij a,⇑, V.A. Barsuk a, V.P. Logvinenko b, A.I. Mescheryakov b, V.G. Otroshchenko a a b
Troitsk Institute for Innovation and Fusion Research, Troitsk, Moscow Reg. 142190, Russia Prokhorov General Physics Institute, Vavilova St. 38, Moscow, 119991, Russia
a r t i c l e
i n f o
Article history: Available online 30 March 2011
a b s t r a c t Experimental results on boronization in plasma shots at the T-11M tokamak and stellarator L-2M are presented. Nontoxic and nonexplosive metacarborane C2H12B10 was used in boron deposition process. Experiments at the tokamak have been carried out in shots with parameters: toroidal field 1–1.2 T, plasma current Ip = 70 kA, average shot duration tp 150 ms and electron density along the central chord ne 2.5 1013 cm 3. The impurities in wall areas have been suppressed. High vacuum characteristics of the discharge chamber were stabilized. Stabilization of a plasma filament has improved. Experiments at the stellarator have been carried out in ohmic heating discharges with parameters: plasma current Ip = 18–20 kA, toroidal magnetic field Bo = 1.25 T, averaged electron density ne = (1.0–1.2) 1013 cm 3, central electron temperature Te = 300 eV and shot duration tp = 30–40 ms. High repeatability of experimental results was achieved. The technology developed provides an opportunity for practical production of renewable boron–carbon coatings using plasma shots in large-scale tokamaks and stellarators. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction In fusion facilities with magnetic confinement the deposition of boron-containing films is widely used for decreasing impurities in the plasma [1,2]. Boronization is carried out in low density helium glow discharges or with the use of plasma electron-cyclotron heating [2]. It is established that the boron film improves conditions of the vacuum chamber surface and plasma facing components. Boron-carbide films decrease the level of heavy impurities (iron, chromium, nickel), they actively absorb oxygen, substantially decrease carbon content and protect the chamber surfaces contacting the plasma. However, the use of a glow discharge with sufficiently low energy characteristics only allows amorphous film thickness of 100 nm, which is eroded during shots of total duration 1–10 s. The typical erosion rate of this film is 3–10 nm/s [2]. Using relatively high density and temperature (40 eV) of the plasma, boron-carbide films with composition close to stoichiometric B4C have been obtained in the PISCES-B facility in the University of California, San Diego [3]. Deposition was performed in plasmas with parameters similar to the DIII-D tokamak divertor plasma. Nontoxic and nonexplosive metacarborane C2H12B10 was used as the working substance. The deposition rate was extremely high and achieved 30 nm/s, approximately one thousand times the rate of film deposition in a glow discharge. Thickness of the deposited layer depended on discharge time and 40 lm thickness was achieved. The coating was dense, without pores and had high hard⇑ Corresponding author. Tel.: +7 496 7510538; fax: +7 495 3345510. E-mail addresses: [email protected] (O.I. Buzhinskij), [email protected] (A.I. Mescheryakov). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.03.038
ness, good adhesion to sample surfaces. The high film deposition rates are apparently linked to the high degree of carborane ionization and dissociation caused by the high density 40 eV PISCES-B plasma compared to electron temperature <1 eV in glow discharge plasmas. Therefore, it is of interest to investigate boronization of the discharge chamber in tokamak and stellarator plasma shots with higher density and electron temperatures. A new method of boron-carborane film deposition was developed for that purpose. The boronization is carried out during the operating pulses of plasma devices when the plasma is heated ohmically and carborane is injected into the plasma during the ohmic discharge. We called this the pulsed boronization method. The present work gives the results of experimental studies of pulsed boronization in ohmic discharge plasmas at the T-11M tokamak [4] and the L-2M stellarator using carborane C2B10H12.
2. Experiments on the T-11M Boronization of the tokamak T-11M chamber during plasma shots was carried out after experiments with the lithium limiter [5]. The placement of the lithium and graphite limiters, diagnostics, transport device with reference specimens and the carborane container in the T-11M tokamak is shown in Fig. 1. The boronization experiments was used nontoxic, nonexplosive metacarborane (C2H12B10) as the working agent. Carborane is a clear crystal substance at room temperature. The molecular structure of carborane is dodecahedral, built from boron and carbon atoms, surrounded by hydrogen atoms. The melting point of metacarborane is 263–265 °C and temperature of sublimation 20 °C. Crystalline
O.I. Buzhinskij et al. / Journal of Nuclear Materials 417 (2011) 586–591
587
Fig. 1. Location of diagnostics (LiI, LiII, Ha, SRX, IR), carborane container, and limiters on the T-11M tokamak.
metacarborane was placed in the glass container that was connected to the high-vacuum electromagnetic valve. The electromagnetic valve was connected to a diagnostic port flange of the tokamak T-11M discharge chamber through a vacuum gate valve. The temperature of the container was regulated by a special heater in the range room temperature to 150 °C. The electromagnetic valve was controlled by the tokamak toroidal field with the time of its opening and closing variable over a wide range. The tokamak plasma current was stabilized at 70 kA, and the toroidal field on axis 1.15–1.2 T. A deuterium and hydrogen mixture (H/D ratio 1:1) was used in plasma shots. The lithium limiter was moved away to the ‘‘shadow’’ during operation with a graphite limiter located at 20 cm from the plasma center. Behavior of the lithium, carbon, boron, deuterium and hydrogen light emission lines was determined by an optical method. Boronization in the T-11M tokamak was carried out after operation with the lithium limiter, without any preliminary induction heating or cleaning of the chamber by a glow discharge. In the preliminary discharges (before boronization) atomic lithium and impurities entered the plasma, produced by ion sputtering from chamber walls. Plasma radiation spectrum (Li, B, C and impurity ion lines and H, D lines) was analyzed by a high resolution optical spectrometer. In Fig. 2 the radiation spectrum of the plasma before boronization is shown by curve 2. There is a bright Li ion line in the plasma radiation spectrum. However, after several shots with carborane the Li line and impurities lines practically vanish
from plasma radiation spectrum (curve 1 in Fig. 2), and a B ion line appears. The plasma radiation spectrum also contains deuterium and hydrogen lines. During lithium experiments the D:H ratio in peripheral plasma was 1:1.3. During boronization the D:H ratio in the peripheral plasma decreased to 1:4 (Fig. 2). When the valve for carborane injection was closed, the B line vanished from the plasma radiation spectrum, at the same time lines from impurities completely vanished. The carborane injection valve was opened for at most 50 ms before the plasma shots and the time of its opening could be varied over a wide range. Change of the valve opening time was easily detected by optical diagnostics of the radiation spectrum during plasma shots. Injection of carborane in the tokamak plasma shots improved the stabilization of plasma filaments, and hydrogen recycling from the chamber walls was decreased. The duration of the tokamak plasma shots increased from tp = 150 to 250 ms at the density of ne = 4.65 1013 cm 3 and up to 350 ms at a density of Ie = 1.3 1013 cm 3. Boronization resulted in a considerable decrease of the volt–second consumption rate (and the corresponding increase of shot duration) (Fig. 3). The impurities near the wall were suppressed and the high vacuum of the discharge chamber was stabilized. Concentration of impurities had decreased by 3–5 times, so the radiation losses decreased by three times and amounted to only 10–20% of the input energy. The boronization during tokamak plasma shots formed a boron–carbon coating on reference specimens and the AISI 321 stainless steel first wall of the discharge chamber (Fig. 4). Scanning electron and optical microscopy examination of coating surface on the samples showed they had a cellular microstructure. The size of cells in the centre of a sample was less than at the periphery. In the centre of a sample the coating thickness was 200 nm, and at the periphery 120 nm. Boronization in tokamak plasma shots occurred during about 50 pulses with total exposure time 8 s, thus the film deposition rate was 25 nm/s, close to the rate of the boron film deposition in the PISCES-B experiments. All plasma shots with carborane had similar values of main plasma characteristics. Vickers microhardness measurements with a load of 10 g showed hardness of the crystalline Si substrate to be H10-250 and microhardness of the boron film H10-600, this indicates the coating produced have ordered structure. (For crystalline boron carbide, B4C, produced by chemical vapor deposition, H101800.)
3. Experiments on the L-2M stellarator
Fig. 2. Plasma radiation spectrum of the T-11M tokamak, before boronization (with lithium limiter) – shot 22641 and at the beginning of boronization – shot 22673.
Glow discharge boronization of the vacuum chamber walls has been carried out on the L-2M stellarator since 2002, and has allowed considerable improvement of the discharge parameters
588
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Fig. 3. Plasma shot parameters before (curve 1), during (curve 2) and after (curve 3) boronization at the carborane temperature of 100 °C: a – current, b – density, c – volt– seconds, d – hard X-ray.
[6]. To carry out pulsed boronization, the same boronization chamber as for the glow boronization was used (see Fig. 5). The only difference is that the output valve was replaced by a pulsed admission valve (5). The boronization chamber was heated to 110 °C and the glass carborane container (2) was heated to 60 °C. The pressure near the pulsed admission valve was maintained at 8 10 2 torr. The pulsed boronization was carried out after preliminary preparation of the stellarator chamber. The magnetic system of the stellarator produces a divertor configuration of the magnetic field with two zero points (see Fig. 6). The flux of charged particles and energy at the X-point of the separatrix are ten times larger than in other locations of the plasma surface. Thus the boron-carbide film is expected to be preferentially formed near the X-point of the separatrix. In plasma shots the plasma-wall interaction occurs precisely at these locations of the chamber surface, so the bor-
on-carbide coating in these locations plays the most important role. As a result of the pulsed boronization cycle which includes the preliminary glow boronization, the fluorescence intensity of carbon and oxygen lines during operation with a hydrogen plasma became 3–5 times lower than before boronization (Fig. 7), and the total radiation losses 2–3 times lower. The loop voltage drops by a factor of 1.5–2 within the constant current ohmic heating. This shows the decrease of plasma effective charge. The plasma density remains constant during all the pulses of ohmic discharge after boronization. It is also worth mentioning that the amount of carbon and oxygen impurities and also the radiation losses remain practically the same after pulsed boronization as after glow boronization. The life test of the deposited boron-carbide film has shown that the film starts to degenerate after approximately 200 stellarator
O.I. Buzhinskij et al. / Journal of Nuclear Materials 417 (2011) 586–591
589
50 ms operating pulses (thus total time of ohmic discharge on the film was 10 s). That is, the film formed by pulsed boronization is more resistant to dispersion than the film formed by the glow boronization. The degradation rate was approximately half that of the glow discharge-formed surface. Apparently boron-carbide film of higher density is formed at those segments of the vacuum chamber surface which are close to the X-point of the separatrix. The ion flux from plasma near Xpoints is 10 times greater than the mean flux from plasma to the segments of the wall which are far from these points. The porous film formed as a result of usual glow boronization remains on the other areas of the vacuum chamber surface. 4. Conclusions
Fig. 4. Boron–carbon coating of the T-11M tokamak discharge chamber.
Experiments on boronization of the T-11M tokamak and the L2M stellarator during plasma shots used metacarborane were successful. As a result of boronization in the tokamak, a film with a
Fig. 5. Boronization system of vacuum chamber on the stellarator L-2M: 1 – silicogel pump, 2 – carborane container, 3 – thermocouple tube, 4 – the cross section of stellarator chamber, 5 – pulsed valve.
Fig. 6. Magnetic surfaces in the stellarator L-2M.
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Fig. 7. Ohmic discharge before (curve 1) and after (curve 2) of pulsed boronization in the stellarator L-2M.
Vicker’s microhardness H10-600 and thickness up to 0.2 lm at a deposition rate 25 nm/s was produced. Impurities in the wall zone were suppressed and a stable, high vacuum was maintained in the discharge chamber. Stabilization of plasma filaments improved, and hydrogen recycling from the chamber walls decreased. Boronization resulted in a considerable decrease of the volt–seconds consumption rate (and, correspondingly, to an increase of shot duration). Plasma shot duration without disruptions increased substantially and at a density of ne = 1.3 1013 cm 3, Ip = 70 kA in tokamak was 350 ms and at a density of ne = 4.65 1013 cm 3, Ip = 70 kA was 250 ms. Pulsed boronization of the vacuum chamber after preliminary preparation by glow boronization was carried out in the L-2M stellarator. Comparison of the results between the pulsed boronization and the glow boronization shows: (1) The amounts of carbon and oxygen impurities in the hydrogen plasma decrease by 3–5 times after boronization and was practically the same after pulsed boronization or glow boronization.
(2) The pulsed boronization applies the boron-carbide film of higher density on the vacuum chamber walls of the stellarator compared to the result with glow boronization. The total time that the wall protection remained effective was two times greater compared to the film formed during the glow boronization. A high repeatability of plasma shot characteristics was achieved in both devices. Proposed use of this technology opens the possibility of practical production of renewable boron carbide coating using plasma shots in large-scale tokamaks and stellarators, such as DIII-D, JET, JT-60 UP, LHD, ITER, and DEMO.
Acknowledgements The authors express their gratitude to Prof. Azizov E.A. for fruitful discussions, permanent interest and support. Work supported by RFBR Grant 11-02-01341-a and in part by RFBR Grant 08-0800504-a.
O.I. Buzhinskij et al. / Journal of Nuclear Materials 417 (2011) 586–591
References [1] J. Winter, H.G. Esser, L. Konen, et al., J. Nucl. Mater. 162–164 (1989) 713. [2] O.I. Buzhinskij, Yu.M. Semenets, J. Fusion Technol. 32 (1) (1997) 1. [3] O.I. Buzhinskij, V.A. Barsuk, V.G. Otroshchenko, J. Nucl. Mater. 390–391 (2009) 996.
591
[4] O.I. Buzhinskij, V.G. Otroshchenko, D.G. Whyte, et al., J. Nucl. Mater. 313–316 (2003) 214. [5] V.A. Evtikhin, I.E. Lyublinski, A.V. Vertkov, et al., The liquid lithium fusion reactor, in: Proceeding of 16th International Conference on Fusion Energy, Montreal, 7–11 Oct. 1996. Fusion Energy 1997, IAEA, Vienna, 1997, vol. 3, pp. 659–665. [6] A.I. Mescheryakov, D.K. Akulina, Plasma Phys. Rep. 31 (6) (2005) 452.